Typically, a flash memory (FM) device has been known as a nonvolatile memory device. Typically, the FM device includes an FM and an FM controller. The FM controller receives a command (e.g., a read command) from a higher-level apparatus with respect to the FM device, and returns a response to the command to the higher-level apparatus. In a case where the command is required to be transmitted to the FM in order to return the response to the command, the FM controller transmits the command (e.g., the read command) to the FM.
The FM includes a plurality of FM chips. Each FM chip is, for example, an NAND-type FM chip. Each FM chip includes multiple blocks. Erasure is performed by the FM controller in units of blocks. “Block erasure” is erasure of data from the block.
Improvement in refinement of FM chips makes FMs have large capacities while causing a possibility of reduction in quality. It is predicted that large increase in the capacity and reduction in the quality of FM increase the command response time of an FM device (a time from reception of the command by the FM device from a higher-level apparatus to return of a response to the command to the higher-level apparatus).
One reason of the prediction of increase in command response time is that increase in erasure time (a time required to perform erasure) is predicted. More specifically, for example, according to at least one of the following (1) and (2), the erasure time is predicted to be increased.
(1) Erasure is performed in units of blocks, while the block size tends to be increased according to large increase in the capacity of FM. The erasure time of a block having a large size is long.
(2) If the quality in FM is reduced, at least one block is prone to deterioration. The deteriorating block has a long erasure time.
The FM chip that contains an ongoing erasure block is in a busy state. The FM controller cannot transmit, to the FM, a command that is designated another block coupled to a same CE (chip enable) line in the FM chip containing the ongoing erasure block.
A plan of suspending erasure by interruption (temporary stop) (e.g., PTLs 1 and 2) can be considered as an improvement plan for reducing the command response time.